An optical interferometer is a device that splits and later recombines coherent optical signals to make a measurement based on information (e.g., phase information) encoded in one or both of the optical signals. Optical interferometers are employed in a number of measurement-related applications, such as making precise measurements of surface topography, measuring precise distances between two objects, or measuring small amounts of motion.
There are a number of high-precision devices based on optical interferometers. One such device is the ring laser gyroscope used in navigation systems for airplanes and missiles. Another such device of is quantum key distribution (QKD) system used in quantum cryptography. This latter device is discussed in detail below and is used as an example application for the present invention.
Quantum Key Distribution (QKD)
QKD involves establishing a key between a sender (“Alice”) and a receiver (“Bob”) by using either single-photons or weak (e.g., 0.1 photon on average) optical signals (pulses) or “qubits” transmitted over a “quantum channel.” The security of the key distribution is based on the quantum mechanical principle that any measurement of a quantum system in an unknown state will modify-its state. As a consequence, an eavesdropper (“Eve”) that attempts to intercept or otherwise measure the exchanged qubits will introduce errors that reveal her presence.
The general principles of quantum cryptography were first set forth by Bennett and Brassard in their article “Quantum Cryptography: Public key distribution and coin tossing,” Proceedings of the International Conference on Computers, Systems and Signal Processing, Bangalore, India, 1984, pp. 175-179 (IEEE, New York, 1984). Specific QKD systems are described in U.S. Pat. No. 5,307,410 to Bennett, and in the article by C. H. Bennett entitled “Quantum Cryptography Using Any Two Non-Orthogonal States”, Phys. Rev. Lett. 68 3121 (1992). The general process for performing QKD is described in the book by Bouwmeester et al., “The Physics of Quantum Information,” Springer-Verlag 2001, in Section 2.3, pages 27-33.
The above-mentioned references by Bennett each describe a so-called “one-way” QKD system based on an extended interferometer having two loops—one at Alice and one at Bob. The loops are used to split and recombine the optical signals. One of the arms in each loop includes a phase modulator so that Alice and Bob can each impart a randomly selected phase to an optical signal traveling therethrough. The modulated optical signals are recombined (interfered) at Bob and measured using single-photon detectors arranged to indicate the overall modulation imparted to the interfered signal. Once a sufficiently large number of optical signals are exchanged, Alice and Bob share their modulation information and establish a secret key between them using known QKD techniques.
In order for the QKD system to function properly, each interferometer loop needs to be formed with great precision so that they have the same optical path length. This ensures that optical signals traveling over different paths of the interferometer meet at the same time and place and interfere. Given that the optical signals travel at nearly the speed of light in an optical fiber, even very small differences in the optical path taken by the two optical signals can lead to the optical signals missing each other and not interfering. The typical laser coherence length used in QKD systems is normally on the order of 10 ps (1 ps=200 mkm, assuming an optical fiber refractive index of 1.5). To assure good visibility (interference), the optical fiber sections that make up the arms of each optical fiber loop must be fabricated to an accuracy of the order of 1 ps (˜200 mkm or 0.2 mm) or better. This is very difficult to do in practice, adds significantly to the overall manufacturing cost of the interferometer, and represents a source of manufacturing errors.